1. Field of the Invention
The present invention relates to a discharge circuit for pulsed laser which performs pulsed laser oscillation by preionizing across main discharge electrodes disposed in a laser medium and performing a main discharge to excite the laser medium.
2. Description of the Prior Art
TEA laser causes a uniform glow discharge across a pair of opposing main discharge electrodes to form an inverted population region necessary for laser oscillation.
To obtain the glow discharge spread to fill the entire main discharge gap, the TEA laser has to cause preionization before starting the main discharge to ionize the entire main discharge gap previously. Especially, an excimer laser has to ionize as many as possible immediately before the main discharge because the electrons in a negative gas have a short lifetime.
Currently, various types of methods using X rays, a spark discharge, a corona discharge or the like are used as a preionizing method. Among them, a method using the corona discharge is extensively used because it is relatively simple and easy and has less contamination of a laser gas.
FIG. 10 shows an equivalent circuit of a conventional capacity shift type magnetic pulse compression discharge device, namely of a discharge circuit for pulsed laser, which employs the corona discharge for the preionization. FIG. 11 shows an example of voltage and electric current waveforms at respective points of the discharge circuit for pulsed laser shown in FIG. 10.
In the discharge circuit for pulsed laser shown in FIG. 10, corona preionization capacitor (hereinafter called the preionization capacitor) Cpp and corona preionization electrode (hereinafter called the preionization electrode) 4 are disposed to preionize in main discharge gap 3 which is formed between a pair of main discharge electrodes 1, 2, and a laser medium in the main discharge gap between the main discharge electrodes 1, 2 is preionized by UV (ultraviolet) light produced by a corona discharge at the preionization electrode 4.
The discharge circuit for pulsed laser shown in FIG. 10 has two-stage magnetic pulse compression circuit utilizing a saturation phenomenon of three magnetic switches AL0 to AL2 each made of a saturable reactor.
In the discharge circuit for pulsed laser shown in FIG. 10, an electric charge is applied from high-voltage power source HV to capacitor C0 through the magnetic switch AL0 and coil L1.
Then, when pulse oscillation synchronizing signal (trigger signal) TR, which is turned on in synchronization with a repetition frequency of a pulsed laser oscillation, is input, main switch SW is turned on (at the moment t0 shown in FIG. 11). When the main switch SW is turned on, electric potential VSW of the main switch SW drops sharply to zero. When time integral (namely, a time integral value of voltage VC0) S0 of voltage difference xe2x80x9cVC0-VSWxe2x80x9d between the capacitor C0 and the main switch SW as voltages at both ends of the magnetic switch AL0 reaches a threshold value which is based on a set characteristic of the magnetic switch AL0, the magnetic switch AL0 is saturated at the moment t1, and electric current pulse i0 flows through a loop of the capacitor C0, the magnetic switch AL0, the main switch SW and capacitor C1.
Duration xcex4 0 in which the electric current pulse i0 starts to flow and becomes 0 (the moment t2 shown in FIG. 11), namely electric charge transfer time xcex4 0 in which the electric charge is completely transferred from the capacitor C0 to the capacitor C1, is based on respective capacitance of the inductance, the capacitor C0 and the capacitor C1 after the magnetic switch AL0 is saturated with loses due to the main switch SW and the like disregarded.
Meanwhile, when time integral S1 of the voltage VC1 of the capacitor C1 reaches the threshold value which depends on the set characteristic of the magnetic switch AL1, the magnetic switch AL1 is saturated and has a low inductance at the moment t3. Thus, electric current pulse i1 flows through a loop of the capacitor C1, capacitor C2 and the magnetic switch AL1. The electric current pulse i1 becomes 0 at the moment t4 after a lapse of predetermined transfer time xcex4 1 which is determined by an inductance after the saturation of the magnetic switch AL1 and the capacitance of the capacitors C1, C2.
When time integral S2 of voltage VC2 of the capacitor C2 reaches a threshold value which is based on a set characteristic of the magnetic switch AL2, the magnetic switch AL2 is saturated at the moment t5. Thus, electric current pulse i2 flows through a loop of the capacitor C2, peaking capacitor CP and the magnetic switch AL2. The flow of the electric current pulse i2 rises voltage Vcp of the peaking capacitor Cp and voltage VCpp of the preionization capacitor Cpp.
Then, when the voltage of the preionization electrode 4 rises to a predetermined preionization start voltage through the preionization capacitor Cpp, a corona discharge is caused at the preionization electrode 4 to flow electric current i3, and the main discharge gap 3 is preionized.
Besides, the voltage VCp of the peaking capacitor Cp rises further with the progress of charging. And, when the voltage VCp reaches a predetermined main discharge start voltage, a laser gas between the main discharge electrodes 1, 2 is undergone an electrical breakdown at the moment t6, and a main electrical discharge is started across the main discharge electrodes 1, 2 to flow electric current i4. The laser medium is excited by the main electrical discharge caused across the main discharge electrodes 1, 2, and laser light is emitted in several nsec.
Then, the voltages of the peaking capacitor Cp and the preionization capacitor Cpp drop sharply owing to the main discharge and return to the states before the charging was started after a lapse of a predetermined period.
Such an electrical discharge operation is repeated by the switching operation of the main switch SW which is synchronized with the trigger signal TR to perform pulsed laser oscillation at a predetermined repetition frequency (namely, a pulse oscillation frequency).
In such a case, because it is determined that the electric charge transfer circuit of each stage which is comprised of the magnetic switch and the capacitor has smaller inductance toward later stages, the pulse compression operation is performed so that the peak values of the electric current pulses i0 to i2 become high sequentially and the electrifying duration becomes shorter sequentially. As a result, an intense electrical discharge is caused across the main discharge electrodes 1, 2 in a short period of time.
An electrical discharge circuit other than the discharge circuit for pulsed laser shown in FIG. 10 is disclosed in Japanese Patent Application No. 9-271207 (Laid-Open Publication No. 11-112300) filed in Japan in the name of the applicant of this patent application.
Density Ne0 of electrons produced by the preionization and its spatial distribution have an influence upon the growth and stability of a glow discharge in the high-pressure laser gas. And, they are included in the factors which exert an influence upon the laser output power, the spatial intensity distribution of the laser beam, the pulse width and the like.
With the increase of the electron density Ne0, the generation and maintenance of the stable glow discharge are facilitated, and the laser output power obtained is also increased.
But, in the above conventional discharge circuit for pulsed laser, the main discharge (namely, the glow discharge) by the main discharge electrodes 1, 2 is started in a state that the preionization in the main discharge gap by the corona discharge is not performed sufficiently, namely in a state before reaching the aforesaid electron density Ne0 with that the stable glow discharge can be caused and maintained. Therefore, the stability and oscillation efficiency of the energy of the pulsed laser light subject to a pulse oscillation are degraded.
In other words, the preionization capacitor Cpp is connected to the peaking capacitor Cp, so that the preionization capacitor Cpp is also charged in synchronization with the charging to the peaking capacitor Cp.
Therefore, emission timing of the corona discharge (preionization discharge) by the preionization electrode 4 depends on the voltage applied to the main discharge electrodes 1, 2, and the duration between the corona discharge by the preionization electrode 4 and the main discharge by the main discharge electrodes 1, 2 becomes short.
Accordingly, the main electrical discharge (namely, the glow discharge) by the main discharge electrodes 1, 2 is started before the aforesaid electron density Ne0 in the main discharge gap 3 reaches an electron density with that the stable glow discharge can be caused and maintained.
As described above, the conventional discharge circuit for pulsed laser has a short duration between the corona discharge by the preionization electrode 4 and the main discharge by the main discharge electrodes 1, 2. Therefore, the main electrical discharge is caused with the preionization in an insufficient state, and the energy stability of the pulsed laser light and the high oscillation efficiency cannot be obtained.
The present invention was achieved in view of the circumstances described above. And, it is an object of the invention to provide a discharge circuit for pulsed laser which can start a main discharge across main discharge electrodes with a main discharge gap between the main discharge electrodes fully preionized.
A first aspect of the invention is directed to a discharge circuit for pulsed laser including a power supply and main discharge electrodes disposed in a laser medium, which causes a main discharge after causing a preionization between the main discharge electrodes, the discharge circuit comprising:
a main discharge capacitor which is connected in parallel to the main discharge electrodes and accumulates electric charges;
a forwarding capacitor which is connected in parallel to the main discharge capacitor and accumulates electric charges supplied from the power supply;
a magnetic switch which is disposed in correspondence with the forwarding capacitor and transfers the electric charges accumulated in the forwarding capacitor to the main discharge capacitor;
a preionization electrode for preionizing between the main discharge electrodes; and
a preionization capacitor connected at one connecting portion thereof with the preionization electrode, for accumulating electric charges for causing a preionization discharge at the preionization electrode,
wherein the other connecting portion of the preionization capacitor is connected to the forwarding capacitor.
A second aspect of the invention is the discharge circuit for pulsed laser according to the first aspect of the invention, wherein the forwarding capacitor is comprised of at least one forwarding capacitor connected in parallel to the main discharge capacitor;
the magnetic switch is disposed in correspondence with the at least one forwarding capacitor and comprised of at least one magnetic switch for transferring the electric charges accumulated in the forwarding capacitor to a next forwarding capacitor or the main discharge capacitor; and
the other connecting portion of the preionization capacitor is connected to a predetermined particular forwarding capacitor among the at least one forwarding capacitor.
A third aspect of the invention is the discharge circuit for pulsed laser according to the second aspect of the invention, wherein the other connecting portion of the preionization capacitor is connected to a final forwarding capacitor or a forwarding capacitor which is disposed on the side of the power supply than the side of the final forwarding capacitor.
The first to third aspects of the invention will be described with reference to FIG. 1 and FIG. 2.
In discharge circuit for pulsed laser 10 shown in FIG. 1, one connecting portion of preionization capacitor Cpp whose other connecting portion is connected to preionization electrode 4 is connected to a junction between capacitor C2 as a forwarding capacitor and magnetic switch AL2.
In such a case, when an electric charge is transferred from capacitor C1 to the capacitor C2, namely when electric current pulse i1 flows through a loop of capacitor C1 as a forwarding capacitor, the capacitor C2 and the magnetic switch AL2 at the time t3 shown in FIG. 2, voltage VC2 of the capacitor C2 and voltage VCpp of the preionization capacitor Cpp are increased because the one connecting portion of the preionization capacitor Cpp is connected to the capacitor C2.
Then, when the voltage of the preionization electrode 4 is increased to a predetermined preionization start voltage through the preionization capacitor Cpp, a corona discharge is caused at the preionization electrode 4 to flow electric current i3, and the main discharge gap 3 is preionized.
In other words, the voltage VCpp of the preionization capacitor Cpp which is charged in synchronization with charging of the capacitor C2 at the time t3 shown in FIG. 2. And, when the voltage of the preionization electrode 4 is increased to a predetermined preionization start voltage through the preionization capacitor Cpp, the main discharge gap 3 is preionized by the corona discharge caused by the preionization electrode 4.
Meanwhile, when the magnetic switch AL2 is saturated at the time t5 shown in FIG. 2, the electric charge accumulated in the capacitor C2 is transferred to peaking capacitor Cp as a main discharge capacitor, and voltage VCp of the peaking capacitor Cp is increased. When the voltage VCp reaches a predetermined main discharge start voltage, a laser gas between the main discharge electrodes 1, 2 is undergone an electric breakdown at the time t6, and the main discharge is started across the main discharge electrodes 1, 2 to flow electric current i4. And, the laser medium is excited by the main discharge caused across the main discharge electrodes 1, 2, and laser light is produced in several nsec.
Thus, the corona discharge is caused at the preionization electrode 4 to preionize according to the voltage VCpp of the preionization capacitor Cpp which is charged at the time t3 shown in FIG. 2, and the main discharge gap 3 is fully preionized before the main discharge is caused at the time t6 shown in FIG. 2.
In the discharge circuit for pulsed laser 10 shown in FIG. 1, the one connecting portion of the preionization capacitor Cpp whose other connecting portion is connected to the preionization electrode 4 is connected to the junction between the capacitor C2 and the magnetic switch AL2 but may be connected to a junction between the capacitor C1 and the magnetic switch AL1.
In such a case, when the electric charge is transferred from the capacitor C0 to the capacitor C1, namely when the current pulse i0 flows through a loop of the capacitor C0, the magnetic switch AL0, the main switch SW and the capacitor C1 at the time t1 shown in FIG. 5, the voltage VC1 of the capacitor C1 and the voltage VCpp of the preionization capacitor Cpp are increased because the one connecting portion of the preionization capacitor Cpp is connected to the capacitor C1.
Then, when the voltage of the preionization electrode 4 is increased to a predetermined preionization start voltage through the preionization capacitor Cpp, a corona discharge is caused at the preionization electrode 4 to flow the electric current i3, and the main discharge gap 3 is preionized.
In other words, the voltage VCpp of the preionization capacitor Cpp which is charged in synchronization with the charging of the capacitor C1 at the time t1 shown in FIG. 5. And, when the voltage of the preionization electrode 4 is increased to a predetermined preionization start voltage through the preionization capacitor Cpp, the main discharge gap 3 is preionized by the corona discharge caused by the preionization electrode 4.
Meanwhile, the laser gas between the main discharge electrodes 1, 2 is undergone the electric breakdown at the time t6 shown in FIG. 5 to start the main discharge across the main discharge electrodes 1, 2 so to flow the electric current i4. And, the laser medium is excited by the main discharge caused across the main discharge electrodes 1, 2, and laser light is produced in several nsec.
Thus, when the corona discharge caused at the preionization electrode 4 to preionize according to the voltage VCpp of the preionization capacitor Cpp which is charged at the time t1 shown in FIG. 5, the main discharge gap 3 is fully preionized before the main discharge is started at the time t6 shown in FIG. 5.
As described above, according to the first to third aspects of the invention, the one connecting portion of the preionization capacitor Cpp whose other connecting portion is connected to the preionization electrode 4 is not connected to the peaking capacitor Cp but to the capacitor C2 before the peaking capacitor Cp or the capacitor C1 two stages before the peaking capacitor Cp, so that the emission timing of preionization can be synchronized with the charging of the capacitor C2 or the capacitor C1. Thus, the main discharge by the main discharge electrodes 1, 2 can be performed in the fully preionized state.
Therefore, the main discharge is readily caused, and the produced main discharge is stabilized. Accordingly, the laser output can be stabilized.
A fourth aspect of the invention is the discharge circuit for pulsed laser according to the first, second or third aspect of the invention, wherein a one-directional circuit element is disposed between the other connecting portion of the preionization capacitor and a forwarding capacitor connected with the other connecting portion in order to prevent an electric current from flowing from the connecting portion to the preionization capacitor.
A fifth aspect of the invention is the discharge circuit for pulsed laser according to the fourth aspect of the invention, wherein the one-directional circuit element is a diode.
A sixth aspect of the invention is the discharge circuit for pulsed laser according to the fourth aspect of the invention, wherein the one-directional circuit element is a saturable reactor which is pre-saturated by a forward current.
Then, the fourth to sixth aspects of the invention will be described.
In the equivalent circuit of the discharge circuit for pulsed laser 30 shown in FIG. 6, diode D is disposed as a one-directional circuit element between the capacitor C2 and the preionization capacitor Cpp.
If the diode D is omitted from the equivalent circuit shown in FIG. 6 (namely, the equivalent circuit shown in FIG. 1), this diode D prevents current pulse ipp from flowing to the preionization capacitor Cpp when the electric charge is transferred from the capacitor C2 to the peaking capacitor Cp as the current pulse i2 flows.
In the equivalent circuit of discharge circuit for pulsed laser 40 shown in FIG. 7, the diode D is disposed between the capacitor C1 and the preionization capacitor Cpp.
When the diode D is omitted from the equivalent circuit shown in FIG. 7 (namely, the equivalent circuit shown in FIG. 4), the diode D prevents the current pulse ipp from flowing to the preionization capacitor Cpp when the electric charge is transferred from the capacitor C1 to the capacitor C2 as the current pulse i1 flows.
Instead of the diode D, a saturable reactor AL3 which is pre-saturated by a forward current may be disposed as a one-directional circuit element between the capacitor C1 or the capacitor C2 and the preionization capacitor Cpp as shown in FIG. 8 or FIG. 9.
The saturable reactor AL3 prevents the current pulse ipp in the same way as the diode D.
Thus, where the one-directional circuit element is not disposed between the preionization capacitor and the forwarding capacitor, the current pulse flowing from the connecting portion to the preionization capacitor can be prevented according to the fourth to sixth aspects of the invention.
Therefore, the voltage of the preionization electrode is increased to the predetermined preionization start voltage through the preionization capacitor which is charged as the current pulse flows, and the second corona discharge (namely, the second preionization) by the preionization electrode can be prevented from occurring.